Belief one’s all-powerful overturned! (5)
I believe the answer is:
dogma
'belief' is the definition.
(synonyms)
'one's all-powerful overturned' is the wordplay.
'one's' becomes 'am' ('one is' can humorously mean 'I am').
'all powerful' becomes 'god' (I can't justify this - if you can you should believe this answer much more).
'overturned' says the letters should be written in reverse.
'am'+'god'='amgod'
'amgod' reversed gives 'DOGMA'.
Can you help me to learn more?
(Other definitions for dogma that I've seen before include "Principles laid down by authority as true" , "Opinion viewed as truth?" , "Incontrovertible set of principles" , "Fixed belief" , "Non-evidence based opinion" .)